A conventional network system will be described with reference to FIGS. 15-17.
As shown in FIG. 15, a conventional network system is made up of a plurality of network terminals. In this network system, at least one of the network terminals is designated as a master terminal (master) for administering the network. The master terminal periodically outputs a synchronization information frame (beacon frame).
The synchronization information frame includes at least a network identifier for identifying the network system, a master terminal inherent number for identifying a sender of the synchronization information frame, a synchronization information frame interval indicating when the next synchronization information frame is sent, and band guarantee period information for specifying network terminals which can transmit information to another network terminal and for guaranteeing a transmission band.
The network terminals (slaves) other than the master terminal receive the synchronization information frame which is periodically sent from the master terminal, and then the network terminals are synchronized with synchronization information frame. Furthermore, in accordance with the band guarantee period information included in the synchronization information frame, each of the network terminals is allowed to carry out data transport only during a band guarantee period corresponding to that network terminal. With this, a band for the data transport between the network terminals is guaranteed.
To the master terminal, the network terminals also make requests for participating in the network system that is administered by the master terminal, and for transmitting a band guarantee period used after the participation in the network. Each time the master terminal receives such requests from the network terminals, the master terminal updates information which relates to the network and is included in the synchronization information frame. Correspondingly, each time the network terminals receive the synchronization information frame, the terminals have to follow an instruction in the synchronization information frame.
Since the master terminal and network terminals operate in such a network system, bands for communications between the network terminals are guaranteed and high-quality data transport for real-time contents such as videos is realized without the intermediary of the master terminal. Such a system has been taken by the Institute of Electrical and Electronics Engineering (IEEE) as a standard for PANs (Personal Area Networks), e.g. IEEE 802.15.3.
However, the above-mentioned system has a following problem: when the master terminal cannot transmit the synchronization information frame, or when a network terminal cannot receive the synchronization information frame, the network terminal operating in accordance with the synchronization information frame cannot be synchronized with the network and thus cannot carry out transmitting.
On the occasion that one of the followings occurs: (i) the transmitting of the synchronization information frame from the master terminal is likely to become inexecutable, (ii) the network is overloaded, and (iii) the network will be terminated, the process of allowing one of the network terminals to takeover the role of the master terminal is generally carried out before the transmitting of the synchronization information frame is stopped. With this process, even if the previous master terminal stops the transmitting of the synchronization information frame, a newly-designated master terminal starts to transmit a synchronization information frame and the remaining network terminals receive that frame, and hence the network is maintained. Note that, there are several ways to determine which network terminal takes over the role of the master terminal. For instance, a network terminal which is, in terms of capabilities, the most appropriate for the master terminal is selected, or the selection is made by the master terminal in advance. For instance, Japanese Laid-Open Patent Application No. 10-308697/1998 (Tokukaihei 10-308697; published on Nov. 17, 1998; hereinafter, Patent Document 1) is an example of the above. In this example, a master terminal supplies clock.
If the transmitting of the synchronization information frame from the master terminal is suddenly terminated for some reason before another network terminal takes over the role of the master terminal, the network terminals cannot receive the synchronization information frame so that the data transport between these terminals cannot be carried out. In this case, a master terminal is newly selected from the network terminals and the network is recovered, so that the data transport between the network terminals, which has been terminated due to the obstruction, is resumed.
To achieve this, a time chart in FIG. 16 showing a process by a new master terminal and a process by network terminals other than the new master terminal, and a flow chart in FIG. 17 shows a process until the data transport is resumed are required.
Referring to FIG. 17, the process until resuming the data transport is described below.
After missing the synchronization information frame (S400), a new master terminal is selected from the network terminals other than the existing master terminal (S401), and the new master terminal starts to transmit a synchronization signal (S402).
A network terminal other than the newly-selected master terminal waits for a synchronization information frame supplied from a master terminal (S403). If the network terminal receives the synchronization information frame from either the new master terminal or the existing master terminal (YES in S404), the network terminal is synchronized with and participates in a network administered by the master terminal from which the synchronization information frame is supplied (S405).
A network terminal, which carried out data transport to another network terminal before the obstruction, searches the network in which the network terminal newly participates for the target network terminal with which the network terminal communicated (S406).
If the target network terminal is not found (NO in S407), there is such a possibility that the target network terminal is in a network administered by another master terminal. On this account, the network terminal removes itself from the currently-participating network and then awaits a synchronization information frame from another master terminal (S408).
Meanwhile, if the target network terminal is found (YES in S407), the network terminal makes a band request to the master terminal (S409). The network terminal keeps making the band request until the master terminal accepts the same (S410). Once a band is allocated, the transmission between the network terminals starts (S411).
In the above-described arrangement, a new master terminal is selected from network terminals other than the previous master terminal, after the transmitting of the synchronization information frame from the previous master terminal (old master terminal) is stopped, as in above-mentioned S401. Being alternative to this, Japanese Laid-Open Patent Application No. 2002-111689 (Tokukai 2002-111689; published on Apr. 12, 2002; hereinafter, Patent Document 2) teaches that, before the master terminal (master) leaves the network, information concerning priorities for determining which network terminal (slave) will be the next master terminal is broadcasted in advance, and the next master terminal is selected in accordance with the information.
Note that, Patent Document 1 discloses a time division digital wireless mobile communication system for communications between mobile stations, which can disperse the loads on the mobile stations engaging in communications, by switching a mobile station (clock master) emitting an electric wave for synchronization at a predetermined timing.
In this time division digital wireless mobile communication system, the clock master makes a clock master switching request to mobile stations in slave mode, when the clock master meets a condition for transmitting the switching request. An example of the condition is such that the capabilities as a clock master diminish. Receiving this switching request, one of the mobile stations in the slave mode, which meets a condition for transmitting a clock master switching response, responds to the switching request. An example of the condition is such that a mobile station is the most suitable for a clock master in terms of capabilities. With this, as soon as the current clock master is relegated to a mobile station in the slave mode, one of the mobile terminals in the slave mode is designated as a new clock master.
Patent Document 2 also discloses a method of generating backup master information for selecting a new master on the occasion that at some point a device acting as a network master becomes no longer able to fulfill the role as the master.
This method of generating backup master information includes the steps of: (a) transmitting connection information from at least one of slaves in a network; (b) when a network master leaves the network, determining an order of priority of at least one of slaves which can act as a backup master, in accordance with the connection information; and (c) broadcasting the order of priority to another slave in the slaves.
However, the above-described conventional arrangement has such a problem that, when the transmitting of the synchronization information from the master terminal is suddenly terminated for some reason in the middle of data transport between the network terminals, the handover of the role of the master terminal as described in Patent Document 1 cannot be carried out, so that the transmitting of the synchronization information frame including band management information stops. As a result, the network terminals are no longer able to receive the synchronization information frame, thereby being impossible to continue the data transport therebetween.
Furthermore, when a network managed using a synchronization information frame malfunctions due to a failure in the previous master terminal and thus the construction of a new network is attempted, the following processes are typically carried out: a process of selecting a new master terminal from network terminals other than the previous master terminal; a process of participating in the network; a process of searching for a connection target; a process of making a band request and acquiring a band. As a result, if these processes take more than predetermined periods of time, real-time data such as video and audio transmitted before the failure is no longer able to reach the target network terminal, and thus the user of the target network terminal cannot enjoy the contents any more.
Moreover, if the previous master terminal still exists concurrently with a newly-selected master terminal, network terminals which communicated with each other may not be able to find each other on the occasion when these network terminals participate in different networks administered by the respective master terminals. In such a case, the network terminal is required to search for another master terminal, participate in the network administered by the same, and search for the target network terminal in the network, until the target network terminal is found. This further delays the resumption of the data transport, thereby resulting in the interruption of real-time data transport such as video and audio broadcast.
Even if the target network terminal is found as a result of the search in the network, a band allocated for the network terminals coupled in the previous network cannot be obtained, once other network terminals make a band request and obtain that band before the network terminals coupled in the previous network make a band request. This also obstructs the resumption the data transport interrupted by the failure.
According to Patent Document 2, the next master terminal is determined in advance so that the process of selecting a new master terminal is not required. However, Patent Document 2 also cannot solve the problems regarding the remaining processes. That is to say, although the priorities of slaves to be selected as a new master are determined in advance, it is still necessary to check the connection to a prospective master in order to recover the network, and this takes time.